German Patent Publication DE-OS 2,641,828 (Muller et al.) published on Mar. 23, 1978, corresponding to U.S. Pat. No. 4,146,091 (Muller et al) issued Mar. 27, 1979, discloses a screw gun of the type described above. Such screw guns use screws that are assembled along a belt which is held in a magazine. The screw holding belt is made of a synthetic material from which the screws are individually ripped by the driving force of the driver blade. When driving a screw, the operator must push the drive motor with a guide frame or support body against the work piece, whereby the support body remains stationary relative to the work piece and the motor and driver blade are displaced toward the work piece guided by the support body.
In operating the known gun, the operator must overcome the force of two springs when he pushes the drive motor and the blade downwardly for the driving of a screw. One spring is a return or reset spring that aids in the returning of the drive motor and driver blade into a starting or rest position. The other spring is a belt feeder spring that operates the feed advance of the screw belt. The feed advance spring is cocked with each downward movement of the drive motor and driver blade. When the motor and blade are lifted again into a starting or rest position the cocked belt feeder spring is released thereby advancing the screw belt in a direction toward a position in which the next screw in the belt is in axial alignment with the driver shaft or blade. The just described apparatus works very satisfactorily but leaves room for improvement with regard to reducing the force that must be exerted by the operator so as to avoid tiring, especially when prolonged use of the screw gun is necessary.